ECU location in 1987 S10 Chevy Blazer

Hello all,

I'm kind of at the end of my ropes here. I recently purchased a 1987 S10
Blazer 4WD with a 2.8L TBI engine. The Service Engine Soon light keeps
coming on and the code is a 13 (o2 sensor).

So far I've replaced the O2 sensor and the temperature sensor. When that
didn't help I hooked up an oscilloscope to the O2 sensor to check the
voltage level which was around 1.3 Volts (one point three volts) From what
I can tell this reading is WAY out of range.

The next step was to hook up the laptop to it via the ALDL port and get some
readings. The O2 sensor voltage reading according to the ECU was 0.455
Volts.

As far as I know, this could only mean one of two things. Either there's a
problem with connectivity to the ECU (i.e. broken wire) or the sensor isn't
grounded properly (1-wire sensor) and the difference between the sensor's
assumed ground and the ECU's ground is .5-.6 volts.

So I was wondering where I could find the ECU in this thing to trace the
purple O2 sensor wire and make sure it has continuity. Any other ideas are
greatly appreciated.

ECU location in 1987 S10 Chevy Blazer

"Archibald J. Cox" <archibaldjcox@...........m> wrote in message
news:XIEEb.175026$I53.6913122@twister.southeast.rr .com...
> Hello all,
>
> I'm kind of at the end of my ropes here. I recently purchased a 1987 S10
> Blazer 4WD with a 2.8L TBI engine. The Service Engine Soon light keeps
> coming on and the code is a 13 (o2 sensor).
>
> So far I've replaced the O2 sensor and the temperature sensor. When that
> didn't help I hooked up an oscilloscope to the O2 sensor to check the
> voltage level which was around 1.3 Volts (one point three volts) From
what
> I can tell this reading is WAY out of range.
>
> The next step was to hook up the laptop to it via the ALDL port and get
some
> readings. The O2 sensor voltage reading according to the ECU was 0.455
> Volts.
>
> As far as I know, this could only mean one of two things. Either there's
a
> problem with connectivity to the ECU (i.e. broken wire) or the sensor
isn't
> grounded properly (1-wire sensor) and the difference between the sensor's
> assumed ground and the ECU's ground is .5-.6 volts.
>
> So I was wondering where I could find the ECU in this thing to trace the
> purple O2 sensor wire and make sure it has continuity. Any other ideas
are
> greatly appreciated.
>
> Thanks,
> Archie
>

Archie,

On my OT truck I hooked a 1.5 volt AA cell up to the wires to the computer.
I made a simple circuit with a few 1k resistors to roughly divide the
voltage in half, then connected the ECM across one resistor. Measured the
voltage on the resistor and it went up when I turned the ignition on. That
told me the computer was "sourcing" some current to the O2 sensor. I can't
tell you if GM uses the same scheme, but I would check the voltage at the O2
connector, with the sensor disconnected, and see if it goes up with the
ignition on. That would check continuity back to the ECM.

I have no idea where the ECM is, but on most older cars and trucks it's in
the passenger compartment. Usually behind one of the kick panels or on the
firewall under the dash. On my newer GM, it's under the hood next to the
right (Passenger's) hood hinge.

ECU location in 1987 S10 Chevy Blazer

Just an update for anyone following the thread looking for handy
information:

It turns out that the problem causing the code 13 was a bad ground for the
O2 sensor. However, the ground issue was not at the body of the sensor as
originally suspected (sensor to the exhaust, exhaust to engine, engine to
ground) The ECU utilizees a wire in the harnes that ties directly to engine
ground as a ground reference for the O2 sensor. That wire was not going to
ground (I suspect it is broken). Once we grounded that wire, everything
seems to be working normally and the O2 sensor voltage is within range.

In the harness to the ECM, the O2 sensor wire is the PURPLE (no tracer) one
in the larger connector and the ground for it is the TAN wire WITHOUT ANY
OTHER TRACER STRIPE (there's also a tan-white and a tan-black which I don't
remember currently what they go to)